Embossed zigzag resistor grid



July 1, 1969 CROFT ET AL 3,453,585

EMBOSSED ZIGZAG RESISTOR GRID Filed March 17, 1967 INVENTORS.

ARTHUR E. CROFT kl ERNEST I4. MOOD/E F/M 7% M FL-H55 United StatesPatent 3,453,585 EMBOSSED ZIGZAG RESISTOR GRID Arthur E. Croft, Weston,Ontario, and Ernest W. Moodie, Downsview, Ontario, Canada, assignors toSquare D Company, Park Ridge, lll., a corporation of Michigan Filed Mar.17, 1967, Ser. No. 624,042 Int. Cl. H01c 3/00 US. Cl. 338-284 4 ClaimsABSTRACT OF THE DISCLOSURE BACKGROUND OF THE INVENTION Field of theinvention A Sheet-metal electrical resistance element having a zigzagshape and composed of a plurality of elongated resistive portionsextending side by side, the alternate adjacent ends being electricallyjoined by integral resistive connecting portions to provide a seriespath through the elongated portions and wherein the elongated portionsof the zigzag element include integral channellike embossments toprovide additional strength and rigidity to the grid structure.

Description of the prior art Resistor banks in common use for thecontrol of electric jmotors comprise an assembly of stamped sheetmetalresistive elements or grids held in parallel-spaced relationship by ametal framework. Examples of such a resistor bank are disclosed inWright Patent No. 2,378,056 issued June 12, 1945, Widdows Patent No.2,639,360 issued May 19, 1953, and Dyer Patent No. 2,647,978 issued Aug.4, 1953. One of the disadvantages of such prior resistor banks is thelack of rigidity of the grids, particularly when they are made ofrelatively thin sheet metal so as to have a relatively high resistance.The lack of rigidity permits beinding of the grids resulting in contacttherebetween under vibrational forces, thermal stresses, or acombination of the two.

A previous attempt to overcome the problem of lack of rigidity ofresistor grids is disclosed in British Patent No. 26 ,252 AD. 1908 toAllen West. In this prior art structure, a zigzag or sinuous grid isprovided with a continuous corrugation which runs throughout the entirepath of the resistance element and which is not oppositely directed inadjacent legs.

In other prior structures, such as illustrated in US. Patent No.2,665,358 to Robert E. DuBois, the adjacent legs of a resistor grid areof opposite curvature to provide increased rigidity and electricalseparation of adjacent legs, but the curvature extends the full width ofeach leg portion and the opposite curvatures gradually merge at thebridging sections so that there are no flat areas at any place in thegrid.

SUMMARY OF THE INVENTION A zigzag sheet metal resistor grid inaccordance with this invention has embossments which, in adjacent legportions of the grid, are oppositely directed with respect to the planeof the grid, each leg portion having a separate and distinct embossmentwhich extends throughout the length of the leg portion but terminatesshort of the resistive section connecting adjacent leg portions so thatthe connecting sections .are fiat and unembossed. This structureprovides a sheet-metal resistor grid which has adequate rigidity and inwhich any flexure or expansion of the grid upon heating is controlled sothat no substantial deformation or buckling occurs, thereby to preventcontact between closely spaced grids in a resistor bank. The control offlexure is facilitated by the fiat unembossed connecting section betweenadjacent legs which provides a break in the embossed pattern to effect ahinge point for opposing flexural forces of adjacent legs.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is an elevational view of aresistor grid in accordance with this invention;

FIG. 2'is a right end view of the grid of FIG. 1;

FIG. 3 is a sectional view taken generally along the line 3-3 of FIG. 1;and

FIG. 4 is a perspective view of a portion of the resistor grid of FIG.1.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring to the drawings, asheet-metal resistor grid 10 is shown as having terminal lug portions 11and 12 at its opposite ends. The lug portions 11 and 12 have respectiveopenings 14 for receiving respective mounting bolts (not shown) of themetal framework of a resistor bank. The grid 10 is stamped from a sheetof metal of relatively high resistance, such as stainless steel sheet,to define a zigzag formation having a plurality of mutually adjacentintermediate leg portions 15 and 16 interconnected by connectingsections 18. The zigzag formation is connected at its ends to the lugportions 11 and 12 thereby providing a sinuous conductive current pathof considerable length extending between the lug portions 11 and 12. Acentral one of the connecting sections 18, identified as 18a, has anopening 14a for receiving a further mounting bolt (not shown). The nextto last leg portions, identified as 15a and 16a, at opposite ends of thegrid 10 are connected to the respective lug portions 11 and 12 byrespective terminal leg portions 19 and 20 which are offset in oppositedirections at 21 and 22, as best illustrated in FIG. 2, from theirassociated lug portions to effect the desired spacing of the grids asexplained in the aforementioned Patent No. 2,378,056 to D. C. Wright.

In forming the grid 10, suitable separate or combined blanking andforming dies may be utilized to cut away portions of the sheet metalthereby to define the lug portions 11 and 12, the leg portionsintermediate leg portions 15 and 16, the terminal leg portions 19 and20, and the connecting sections 18; to form the offsets 21 and 22; andto form reverse embossments in the 'leg portions 15, 16, 19 and 20 inaccordance with this invention.

The intermediate leg portions 15 bossments 24 and 25, respectively,being convex and the embossment 25 being concave with respect to theside of the grid 10 seen in FIG. 1. The embossments 24 and 25 .arecentrally disposed along the longitudinal axes of their respectiveintermediate leg portions 15 and 16 and are narrow and shallow relativeto the width of the leg portions 15 and 16.

The sections 18 connecting the adjacent leg portions 15 and 16 are notprovided with embossments. Thus the embossments 24 and 25 do not extendthroughout the length of the sinuous path but angle slightly toward theconnecting sections as at 26 and terminate at areas and 16 have emtheembossment 24 27 and 28, respectively, so as to leave the connectingsections 18 totally fiat. Because the embossments 24 and 25 face inopposite directions relative to the plane of the grid, any tendency forthe grid to distort rmaterially in one direction under thermal stress isneutralized by the tendency of the adjacent intermediate leg portions tobend in opposite directions. The control of the distortion appears to befacilitated by the flat unembossed connecting section between adjacentlegs which provides a break in the embossed pattern to effect a hingepoint for opposing fiexural forces in adjacent legs.

The terminal leg portions 19 and 20 are similarly embossed at 29 and 30,respectively, the embossment 29 being concave and the embossment 30being convex with respect to the side of the grid seen in FIG. 1.

It has been found that the relatively shallow and narrow embossments 24,25, 29, and 30 impart adequate rigidity to the grid to prevent contactbetween closely spaced grids under high thermal stresses and vibrationalforces. Because the vibration is controlled by the em- Ibossments andthermal flexure is controlled by the cooperation of the oppositelydirected embossments .and the fiat sections connecting adjacent legportions, grids in accordance with this invention have significantlybetter operating characteristics than those of the prior art.

We claim:

1. In .a generally flat zigzag sheet-metal resistor grid having aplurality of elongated generally-flat intermediate leg portionsextending generally side-by-side in spaced-apart edgewise relationship,the alternate adjacent ends of said intermediate leg portions beingjoined by integral connecting sections to provide a continuous currentpath therethrough, a plurality of embossments respective to theintermediate leg portions, each embossment being disposed along thelongitudinal axis of its respective intermediate leg portion and beingnarrow and shallow relative to the width of its intermediate legportion, the improvement comprising the embossments of adjacentintermediate leg portions being oppositely directed with respect to theplane of the grid, and each embossment extending throughout the lengthof its respective intermediate leg portion and terminating short of saidconnecting section so as to leave said connecting section essentiallyfiat and unembossed.

2. A resistor grid according to claim 1 wherein the end portions of eachembossment curve toward .a connecting section joining adjacentintermediate leg portions and wherein the intermediate portion ofeachembossment is essentially straight throughout its length.

3. A generally fiat zigzag sheet-metal resistor grid comprising .a pairof spaced-apart lug portions and a plurality of spaced adjacentintermediate leg portions disposed in a common plane intermediate saidlug portions, said intermediate leg portions being elongated andgenerally flat, connecting sections integrally conmeeting the alternateadjacent ends of said intermediate leg portions, a first terminal legportion joining one of said lug portions to the intermediate leg portionnext adjacent said one lug portion, a second terminal leg portionjoining the other of said lug portions to the intermediate leg portionnext adjacent said other lug portion, at least one of said terminal legportions having means offsetting its associated lug portion from saidcommon plane, said intermediate leg portions, .said connecting sectionsand said terminal leg portions providing a continuous resistive paththrough the grid between said lug portions, a plurality of embossmentsrespective to the intermediate leg portions, each embossment beingdisposed along the longitudinal axis of its respective intermediate legportion and being narrow and shallow relative to the width of itsintermediate leg portion, the embossments of adjacent intermediate legportions being oppositely directed with respect to said common plane,and each embossment extending throughout the length of its respectiveintermediate leg portion and terminating short of said connectingsection so as to leave said connecting section essentially flat andunenrbossed.

4. A resistor grid according to claim 3 wherein each of said terminalleg portions has means olfsetting its associated lug portion from saidcommon plane and said lug portions are oflset in opposite directionsrelative to said common plane.

References Cited UNITED STATES PATENTS 1,233,191 7/1917 Collins 338-2842,665,358 1/1954 DuBois 338284 X FOREIGN PATENTS Ad. 26,252 1908 GreatBritain.

E. A. GOLDBERG, Primary Examiner.

L. H. MYERS, Assistant Examiner.

US. Cl. X.R. 338287

